A solar cell basically consist of a thin silicon wafer having a single large area of typically a PN junction which covers one side of the wafer facing the source of light. The photo-excited charge carriers flow to a front side metal contact and a back side metal contact, respectively, the former having a geometry such that substantially maximum collection of the charge carriers is accomplished, while the area of the metal contact covering the front surface of the cell and thereby blocking the light is minimized. The described solar cell is monofacial, i.e. the solar cell is only photo-active with respect to light impinging on the front side of the cell. By additionally providing a thin junction layer on the back side of the cell, this too may be made photo-active to form a bifacial solar cell. The solar cells may either have an intrinsic, P or N type substrate, while the doped layer or layers may either be N.sup.+ or P.sup.+.
It is known in the art to use an oxide containing dopant. The U.S. patent specification 4,101,351 describes how an oxide is grown on a crystal, following which parts of the crystal are exposed again to an oxide to define the active areas. This exposure typically takes place by adding to this oxide coated crystal a mask layer resistant to hydrofluoric acid, whereby the oxide layer will be etched away with hydrofluoric acid in the areas where the crystal is not coated with a photoresist layer. The crystal areas thus exposed can then be doped after removal of the photo-resist layer by diffusion of a suitable substance into the crystal at a high temperature, typically in the region about 1000.degree. C. The dopant source material may be added by depositing on the exposed area of the crystal a layer of e.g. silica containing dopant source material. No undesirable auto-doping will take place since the rest of the crystal is protected with the oxide layer. The oxidation, photo-resist masking, etching;and doping process can be repeated several times so that the crystal will have the desired number of doped areas. The stated method has several shortcomings, including e.g. many complicated process steps, such as growing of the oxide on the surface of the crystal, which requires a temperature treatment about 1000.degree. C., as well as requirements concerning pure surroundings in either pure oxygen or water vapour. Exposure of parts of the crystal surface normally requires the use of hydrofluoric acid which is an extremely difficult chemical to work with because of its toxicity.
The object of the invention is to provide a method of making doped areas on semiconductor components, preferably solar cells.